mu-scm: add support for references

Message method + test + docs.

scm/mu-scm.texi

@@ -624,6 +624,19 @@ For example:
 => #f
 @end lisp
 
+@deffn {Scheme Procedure} references message
+@end deffn
+Get the list of references (message-ids of related messages) for this message.
+This combines the @t{References} and @t{In-Reply-To} fields, from oldest to the
+immediate parent. Returns @code{#f} if there are no references.
+
+@lisp
+(references msg)
+=> ("439C1136.90504@@euler.org" "4399DD94.5070309@@euler.org"
+    "20051209233303.GA13812@@gauss.org" "439B41ED.2080402@@euler.org"
+    439A1E03.3090604@@euler.org" "20051211184308.GB13513@@gauss.org")
+@end lisp
+
 @c @deffn {Scheme Procedure} sexp message
 @c @end deffn
 @c Get the message's s-expression.